A work unit (3)
I believe the answer is:
erg
This clue is a double definition.
'a work' is the first definition.
(an erg is a unit of work done)
'unit' is the second definition.
(erg is a kind of unit)
(Other definitions for erg that I've seen before include "Unit of work (phys.)" , "Amount of work" , "Unit of work; dunes area" , "Unit of measurement of work" , "shifting sand" .)